A wellscreen may be used on a production string in a hydrocarbon well and especially in a horizontal section of the wellbore. Typically, the wellscreen has a perforated basepipe surrounded by a screen that blocks the flow of particulates into the production string. Even though the screen may filter out particulates, some contaminants and other unwanted materials can still enter the production string.
To reduce the inflow of unwanted contaminants, operators can perform gravel packing around the wellscreen. In this procedure, gravel (e.g., sand) is placed in the annulus between wellscreen and the wellbore by pumping a slurry of carrier fluid and gravel down a workstring and redirecting the slurry to the annulus with a crossover tool. As the gravel fills the annulus, it becomes tightly packed and acts as an additional filtering layer around the wellscreen to prevent the wellbore from collapsing and to prevent contaminants from entering the production string.
Ideally, the gravel uniformly packs around the entire length of the wellscreen, completely filling the annulus. However, during gravel packing, the slurry may become more viscous as carrier fluid is lost into the surrounding formation and/or into the wellscreen. Sand bridges can then form where the fluid loss occurs, and the sand bridges can interrupt the flow of the slurry and prevent the annulus from completely filling with gravel.
As shown in FIG. 1, for example, a wellscreen 30 is positioned in a wellbore 14 adjacent a hydrocarbon bearing formation. Gravel 13 pumped in a slurry down the production tubing 11 passes through a crossover tool 33 and fills an annulus 16 around the wellscreen 30. As the slurry flows, the formation may have an area of highly permeable material 15, which draws liquid from the slurry. In addition, fluid can pass through the wellscreen 30 into the interior of the tubular and then back up to the surface. As the slurry loses fluid at the permeable area 15 and/or the wellscreen 30, the remaining gravel may form a sand bridge 20 that can prevent further filling of the annulus 16 with gravel.
To overcome sand-bridging problems, shunt tube systems have been developed to create an alternative route for gravel around areas where sand bridges may form. Shunt tube systems are used frequently in gravel packing horizontal, open hole wells. The system uses transport tubes and pack tubes placed along a wellscreen to divert gravel pack slurry past premature bridging or obstructions in the wellbore. The tubes allow the gravel pack slurry to continue to gravel pack the well further downhole. Current external shunt tube systems used for open hole gravel packing operation may have two transport tubes and two pack tubes that provide individual flow paths for the gravel pack slurry. These tubes are located external to the sand screen.
For example, a gravel pack apparatus 100 shown in FIGS. 2A-2C positions within a wellbore 14 and has shunts in the form of transport tubes 140 and pack tubes 150 for creating the alternate route for slurry during a gravel pack operation. The pack tubes 150 have nozzles 152 for exiting of the slurry. As before, the apparatus 100 can connect at its upper end to a crossover tool (33; FIG. 1), which is in turn suspended from the surface on tubing or workstring (not shown).
The apparatus 100 includes a wellscreen assembly 105 having a basepipe 110 with perforations 114 as described previously. Disposed around the basepipe 110 is a screen 120 that allows fluid to flow therethrough while blocking particulates.
The transport and pack tubes 140, 150 are disposed on the outside of the basepipe 110 and can be secured by end rings (not shown). As shown in the end view of FIG. 2A, centralizers 132 can be disposed on the outside of the basepipe 110, and a tubular shroud 135 having perforations 137 can protect the transport and pack tubes 140, 150 and the wellscreen 105 from damage during insertion of the apparatus 100 into the wellbore 14.
At an upper end (not shown) of the apparatus 100, each transport tube 140 can be open to the annulus 16 or may be in fluid communication with another transport tube of another wellscreen joint. Internally, each transport tube 140 has a flow bore for passage of slurry. The slurry can be diverted to the pack tubes 150, which have the nozzles 152 disposed at ports in the sidewall of each pack tube 150 to allow the slurry to exit the pack tube 150. As shown in FIG. 2C, the nozzles 152 can be placed along the pack tube 150 so each nozzle 152 can communicate slurry from the ports and into the surrounding annulus 16. As shown, the nozzles 152 are typically oriented to face toward the wellbore's downhole end (i.e., distal from the surface) to facilitate streamlined flow of the slurry therethrough.
In a gravel pack operation, the apparatus 100 is lowered into the wellbore 14 on a workstring and is positioned adjacent a formation. A packer (18; FIG. 1) is set, and gravel slurry is then pumped down the workstring and out the outlet ports in the crossover tool (33; FIG. 1) to fill the annulus 16 between the wellscreen 105 and the wellbore 14. Because the transport tubes 140 are open at their upper ends, the slurry can flow into both the transport tubes 140 and the annulus 16, but the slurry typically stays in the annulus 16 as the path of least resistance at least until a bridge is formed. As the slurry loses liquid to a high permeability portion 15 of the formation and the wellscreen 105, the gravel carried by the slurry is deposited and collects in the annulus 16 to form the gravel pack.
Should a sand bridge 20 form and prevent further filling below the bridge 20, the gravel slurry continues flowing through the transport tubes 140, bypassing the sand bridge 20 and exiting the various nozzles 152 on the pack tubes 150 to finish filling annulus 16. The flow of slurry through one of the transport tubes 140 is represented by arrow 102.
As can be seen from the above example, the top end ring for an open hole external shunt tube system can secure the transport tubes 140 and pack tubes 150 mechanically to the basepipe 110. In some arrangements, the top end ring can provide a conduit for the fluid to exit the transport tube 140 and to enter the pack tube 150. Because the gravel pack slurry is pumped at elevated pressures through the shunt tube assembly and is a sand-laden, abrasive, and highly erosive, the flow of slurry can erode and damage components, such as such top end rings.
As is typical, the external shunt tube system as depicted in FIG. 2A is eccentric so that the centerline of the shunt tube system is offset from the centerline of the basepipe and the screen. Each of the transport tubes is independent of the other transport tube. If one of the transport tubes plugs, then there is no recovery mode, and operable use of half the system is lost. This increases the possibility of a failed gravel pack operation.
Historically, the problem has been addressed by using an internal shunt tube design, such as disclosed, for example, in US 2005/0028977. The internal shunt tube design places transport tubes and pack tubes internal to the sand screen. The internal shunted system is a concentric system in which the basepipe, the screen, and the shunt system all share the same centerline. A sealed housing is provided over the connection from the pin end of one screen to the box end of the next screen. The concentric annulus formed by this sealed housing and the basepipe and coupling allows for commingling of the gravel pack slurry at the connection of each joint.
Although such a concentric shunt system may be useful, it may require a complicated set of components and assembly steps when running wellscreen in hole. Operators are continually striving to simplify assembly steps in running wellscreens and shunt systems in hole, while still meeting the demands for gravel packing horizontal, open hole wells of extended length.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.